The present invention relates to a ferroelectric film formation solution for forming a ferroelectric film on a substrate and a method for forming a ferroelectric film using the solution. Particularly, the present invention relates to a ferroelectric film formation solution which has neither crystallization nor gelation when forming a ferroelectric film containing a II group element in the periodic table such as Mg, Ca, Sr, or Ba and which can form a stable ferroelectric film on a substrate such as sapphire, titanium, or platinum. The present invention also relates to a method for forming a ferroelectric film using the ferroelectric forming solution.
Lead zirconate titanate (PZT) which is a complex oxide of Pb, Zr, and Ti and lead lanthanum zirconate titanate (PLZT) which is a complex oxide of Pb, Zr, Ti, and La have high dielectric capability, ferroelectricity, piezoelectric effect, and pyroelectricity. The thin films of these complex oxides are therefore used for semiconductor memories such as a DRAM (memories using paradielectrics with high dielectric constant such as (Ba,Sr)TiO.sub.3) and FRAM (memories using ferroelectrics such as PZT), capacitors, sensors, and actuators.
In recent years, ferroelectrics containing a small amount of Nb, Fe, Mn, Al, Sn, Bi, Sb, Ba, Ca, Sr, and the like have been studied to improve the characteristics of PZT or PLZT. These ferroelectrics are reported, for example, in Journal of American Ceramic Society, Vol 77, pages 2620-2624 (1994), Japanese Patent Application Laid-Open (JP-A) No. H1-260870, and the like.
Generally, an organic metal compound such as an alkoxide of a metal which is a component of ferroelectrics is reacted in an organic solvent. Then a chelating agent is added to the mixture to improve the stability of a solution, or the mixture is hydrated. And such a solution is widely used for forming the ferroelectrics.
In such a preparing method, however, a solution for forming a thin film of PZT or PLZT containing a II group element in the periodic table such as Mg, Ca, Sr, or Ba tends to crystallize or gel. Even when these phenomena are not produced, the viscosity of the solution changes with time. If the viscosity of the solution increases, the solution cannot be uniformly applied. Also, in the case where the viscosity of the solution increases or decreases, the film thickness when a film is formed varies, giving rise to the problem that it is very difficult to prepare a solution for forming a stable thin film.